[htmltmpl] RFC: Conditional TMPL_INCLUDE

[Mark F. <mar...@ea...>]

I just had a need to conditionally include one of a few templates dependnig
on the value of a TMPL_VAR. One of the includes (which would not have been
loaded) did not exist, but H::T died because it couldn't locate that file. I
realized H::T probably loads everything at compile time and applies the
conditions at the time the output method is called.

This makes sense since caching would be screwed up if it had to keep
reloading conditional includes with every output.

However, I still feel like I have a need to conditionally load included
templates and I know which ones they are at compile time. It's not going to
change over repeated outputs called on the same cached template. I'm faced
with either structuring the template includes differently (making it uglier
and some duplicated HTML). Or, let it load all the stuff I know I don't need
at compile time and use a TMPL_VAR (the same value for the life of the
template) to tell the output method which included fragment to output.

Question: Does this lead to the idea that it would be useful to have
variables used only at compile time? When I tell it to load a template, why
can't I give it variable name=value pairs to apply *only* at compile time?

I know everyone's a fanatic about efficiency. But, couldn't this lead to
more efficent template processing? If you know you have static values used
for the life of the loaded template, wouldn't it be more efficient to apply
them at the time the template is loaded? Instead of every time the output
method is called?

Examples of such life-of-the-template variables might be the language and
encoding variables used to specify the language being used within that
template? Or, a variable used to pick a navigation side-bar based upon
system-wide parm? (Customized navigation bars for seasonal occasions? Where
the CGI decides at runtime which one it wants loaded based upon the time of
year?)

In my case, all my pages share a common sidebar layout, but different
sidebar choices. I want to nest the set of choices within the common sidebar
layout and let each page (different CGI::App modules) specify which set of
choices should be loaded from that common sidebar layout. But, to do that,
it has to load all of them at compile time and then reevaluate the decision
for each output of the template. When, it's not going to change for the life
of that loaded template.

It's not the end of the world. But, it seemed like a potential opportunity
to make H::T a little more efficient by allowing some evaluation criteria to
be performed at compile time.

Am I totally off base?

Thanks,
Mark